It is often uneconomical or impractical to capture and transport natural gas in a special pipeline from the wellhead to a refinery. In these cases the gas is flared off, or burned. It is thus wasted, both as a source of energy and as a valuable commodity.
In those instances where a pipeline is built, other problems develop. Natural gas at the wellhead usually contains impurities such as carbon dioxide (CO2), and hydrogen sulfide (H2S). When dissolved in water, CO2 is known as carbonic acid. Similarly, H2 becomes hydrosulfuric acid. Either of these acids will cause corrosion problems in pipelines and related equipment during transportation of natural gas.
The natural gas product at retail comprises almost pure methane, but raw natural gas from the wellhead contains a variety of contaminants. In addition to the CO2 and H2S, other gases such as nitrogen and carbon dioxide may be found in oil and gas wells. Solid impurities include sand and dirt from the reservoir; and scale and corrosion products from the piping. The wells produce a mixture of hydrocarbon gas, condensate, or oil; and water with dissolved minerals such as salt. The oil and gas is processed to separate these components.
Natural gas condensate is a mixture of hydrocarbon liquids that are gaseous components of the natural gas produced by natural gas fields. It condenses from the raw gas if the temperature drops below the dew point. The condensate gas includes mostly ethane, propane, butane and pentanes. These components are separated and sold.
Membranes are often used to separate raw gas into components. Each component permeates through the membrane at a different rate. The components such as CO2 and H2S and water will permeate faster through the membrane than components such as N2 and methane. Thus, the feed stream is separated into retentate, which is mostly methane-rich natural gas and is depleted of heavy hydrocarbons; and permeate, which includes CO2, H2S, water, and heavier hydrocarbons described above as condensate gas.
Membranes are typically non-porous polymeric films. The most commonly used polymers for this purpose are polycarbonate, polysulfone, polyimide, and cellulose acetate. Membranes are produced in various configurations, such as flat sheets, spiral wound sheets, or hollow fibers. Hollow fibers are preferred due to having the highest packing density, meaning greatest membrane area per unit volume.
Some limited processing of raw natural gas is often carried out at the well site. However, the complete processing of natural gas is typically carried out at a centralized process plant, or refinery. Gas from the wellhead is transported by pipeline or tanker vehicle to the refinery.
There are benefits for oil and gas companies to utilize natural gas on site to power engines, rather than trucking in diesel fuel or gasoline. Such engines, for example, would power drilling rigs or pump trucks during fracking. Using refined products, such as LNG or CNG, requires offsite processing, trucking and specialized equipment to deliver the fuel to the site. On site natural gas is source or field gas from a well or pipeline. The benefits include cost savings, employee safety, and less environmental disturbance.
Natural gas engines maintain the best performance and require the least maintenance when utilizing a dry, consistent BTU gas delivered at an optimal pressure and temperature. Various engine manufacturers spec different ideal BTU ranges but typically 1000 to 1100 BTU is the prime range that balances horsepower required to do the job with engine and exhaust heat that causes engine and maintenance issues. It is not economical to develop a pipeline infrastructure to a well pad for pre-processed gas to power engines.
Accordingly, there is a need to provide a method and accompanying apparatus for natural gas processing that is transportable to the wellhead site, and that is self-contained, and is capable of processing of raw natural gas of varying quality found at the gas source.
There is a further need to provide a method of the type described, and that can selectively process either low pressure or high pressure source gas.
There is a still further need to provide a method of the type described, and that is capable of being powered by fuels recovered at the wellhead.